vos: aserver is private

The server specified on the command line is used directly to
initialise the attributes structure. Move the variable so it's
local to the block which uses it, and remove the function-wide
initialiser.

Caught by clang-analyzer

Change-Id: Ia636fae2ed505f6958cefbfbaa72ef7e37edec98
Reviewed-on: http://gerrit.openafs.org/9165
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>